Applicants' invention relates to devices that enable conventional telephones to communicate through cellular radiotelephone networks.
Wire-line telephones control a standard land-based public switched telephone network ("PSTN") by sending dialing signals, which include dialed digits, through pairs of copper wires to a switch in a central office of the telephone company. The electrical characteristics of the copper wire pairs are well specified, and thus rotary as well as touch-tone wire-line telephones can be accommodated. In this application, "rotary telephone" will be understood to refer not only to a conventional telephone set having a dialing disk but also to any device that produces signals similar to those produced by such a telephone set.
When a wire-line telephone is used to call another wire-line telephone, the dialing and voice or data signals pass through a telephone channel that has a well-defined electrical bandwidth (typically, 300 hertz (Hz) to 3400 Hz) and some amount of attenuation. Sometimes a call passes through a radio connection provided by a cellular radiotelephone network. The caller may be completely unaware that a radio link is being used. In a cellular radiotelephone system, a base station (BS) communicates with a number of mobile stations (MSs) via one or more radio channels. Some current cellular mobile telephone systems use analog frequency modulation (FM) of a radio-frequency (RF) carrier signal to transmit speech and other information.
One principal standard is the Advanced Mobile Phone Service (AMPS) system in the U.S. that uses FM having a spacing of thirty kilohertz (30 KHz) between carrier signals having frequencies of about 880 megahertz (MHz). Some of the characteristics of the AMPS system are specified by the EIA/TIA-553 standard published by the Electronic Industries Association and Telecommunications Industry Association (EIA/TIA). Another principal standard is the Total Access Communications System (TACS) in the United Kingdom that uses spacings of 25 KHz between 880 MHz carrier signals. Yet another standard is the Nordic Mobile Telephone (NMT) system in Scandinavia that uses FM having spacings of 12.5 KHz between carriers in the 450 MHz and 900 MHz bands.
In general, a radio channel is a bi-directional radio transmission path between two transceivers, and thus the channel comprises two carriers having different frequencies, one for transmission and one for reception by a given transceiver. In the standard systems, the frequency spacing between the two carriers of any radio channel is 45 MHz.
One difference between a standard rotary wire-line telephone and a touch-tone wire-line telephone or a cellular telephone is that the rotary telephone still uses pulses to indicate dialed digits rather than tones that are suitable for transmission through the telephone channel. The pulses are generated by successively opening and closing a switch, thereby making and breaking a connection from a local battery of the telephone company. The time duration of each pulse is between about forty milliseconds (40 ms) and about 60 ms, depending on the applicable national standard.
These pulses do not have electrical characteristics that enable them to pass through a PSTN or cellular radio network without severe distortion. As a result, the digits dialed on a rotary telephone may be nearly unintelligible when they arrive at their destination.